This is a proposal from 5 investigators in the School of Biological Sciences at the University of Texas at Austin requesting purchase of a confocal/two-photon microscope. These investigators, the Major Users, will use this state-of-the-art microscope to facilitate and expand their NIH-funded investigations. These investigations include an NEI-funded study of normal binocular integration in visual cortex and its disruption by strabismus, an NIDCD-funded study of membrane conductances, synaptic placement, and cellular morphology underlying precise temporal coding of information in brainstem auditory nuclei, an NINDS-funded study of the signaling mechanisms specifying the fates of individual neurons in the developing midbrain, an NEI-funded study of choroid fissure closure and its genetic regulation in the zebrafish eye, and an NINDS-funded study of mechanisms by which glia shape synaptic development at the neuromuscular junction. The instrument will allow several experiments that are not possible now and accelerate the progress of others, including vital imaging of the movements of individual fluorescent-protein-labeled cells in living animals, optical highlighting of individual cells expressing transgenically-encoded photoswitchable GFP, in vitro physiology of brain slices using optical and electrophysiological measurements, optical identification of the inputs to visual cortex that are known to change following altered visual experience and the examination of changes at individual synapses, reconstruction of the entire arbors of neurons recorded and marked in brain slices, examination of the consequences of altering cell-cell signaling on cell polarity and cell fate during development, and high resolution, 3D reconstruction of individual cells and their molecular components using multi-colored fluorescent probes. The application documents why the needs for this instrument cannot be met by existing instrumentation, presents a detailed plan for the management of the microscope and for training of individuals who use it, and outlines the commitment of the University for provision of space, additional accessory equipment, and maintenance of the instrument. The application identifies 6 Minor Users who will also make use of the instrument and documents how new users can be included. The research will advance our understanding of how individual neurons are specified in development, the principles by which neurons are constructed and connected so as to precisely encode and transmit information, and how synapses are maintained and remodeled. Each of these projects increase our understanding of how genetic alterations, impaired experience, aging, and disease produce pathology and define mechanisms that can be targets of therapy.